THE VAPOR PRESSURES AND THERMAL 
PROPERTIES OF POTASSIUM AND 
SOME ALKALI HALIDES 


BY 


ERNEST FRANKLIN FIOCK 


B.S.—University of Illinois, 1923 
M.S.—University of Illinois, 1924 


A DIGEST OF A THESIS 


SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS 
FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN CHEMISTRY 
IN THE GRADUATE SCHOOL OF THE UNIVERSITY 
OF ILLINOIS, 1926 


THE LIBRARY OF (HE 


ie RF GET REE 


DEC 2 © 1926 


Reprinted from the Journal of the American Chemical Society, 
48, 2522 (1926). 


iO 
| 


( 5439 7 


{Reprint from the Journal of the American Chemical Society, 48, 2522 (1926). ] 


[CONTRIBUTION FROM THE CHEMICAL LABORATORY OF THE UNIVERSITY OF ILLINOIS] 


THE VAPOR PRESSURES AND THERMAL PROPERTIES OF 
POTASSIUM AND SOME ALKALI HALIDES 


By ErRnEs?T F. Frock! anp WortH H. RODEBUSH 


RECEIVED JUNE 10, 1926 PUBLISHED OCTOBER 6, 1926 


The physicists within the last few years have made a large number of 
measurements of energy changes in atoms and molecules by ionization 
and spectroscopic methods and have postulated various electron dis- 
placements and dissociations as accompanying these changes. It is de- 
sirable to obtain direct thermal data for these postulated reactions for pur- 
poses of checking against the calculations of the physicists and verifying 
their postulates. Thus, if the reaction Na + Cl—» NaCl in the vapor 
state consists simply of the transfer of an electron from the sodium to the 
chlorine, the heat of this reaction should be calculable from the ionization 
potential of sodium and the electron affinity of chlorine. The comparison 
of this calculated heat with the more directly determined quantity should 
decide whether the mechanism of the reaction is the simple one postulated 
above. 

It is true that the physicists have not been able to agree upon a value 
for the electron affinity of chlorine but it seems probable that a satisfactory 
value will be established in the near future. On the other hand, the classical 
calculation of this sort made by Foote and Mohler? for gaseous hydrogen 
chloride now appears to be in fortuitous agreement only with thermal 
data since Barker and Duffendack? have shown that the hydrogen chloride 
molecule does not dissociate on ionization as they supposed. In fact, 
it seems likely that no chemical reaction is so simple as the elementary 
process that the physicist brings about in his measurement. ‘Thus the 

1 This communication is an abstract of a thesis submitted by Ernest F. Fiock in 
partial fulfilment of the requirements for the degree of Doctor of Philosophy in Chem- 
istry at the University of Illinois. 

2 Foote and Mohler, ‘‘ The Origin of Spectra,’’ Chemical Catalog Co., New York, 


1922, p. 185. 
3 Barker and Duffendack, Phys. Rev., 26, 339°(1 925). : 


~~ 


Oct., 1926 SOME PROPERTIES OF POTASSIUM 2020 


Einstein law of the photochemical equivalent applies only to the removal 
of an electron from an atom, a process which does not seem to correspond 
to any chemical reaction. Hence, the failure of any simple radiation theory 
of chemical action is not surprising. Nevertheless, there is still hope that 
thermal quantities may be calculated from the data of the physicist with 
high accuracy as, for example, heats of dissociation from band spectra. 

On the other hand, the thermal data are far from complete. In his re- 
cent calculations of the energy of salt vapors Latimer* was compelled to 
estimate heat quantities of large magnitude in the absence of satisfactory 
data. In the case of the alkali metals and alkali halides 
the heats of vaporization are the most important quantities 
lacking. These can be calculated from vapor-pressure 
data of sufficient accuracy. Kroner’s® data on potassium 
do not appear to be very concordant. Von Wartenberg,® 
Ruff,’ Maier® and Jackson and Morgan® have made meas- 
urements on the alkali halides but the agreement is not 
good. Since the method devised by Rodebush and Dixon!° 
for the measurement of vapor pressures at high tempera- 
tures promised to give accurate results on these substances, 
it has been used in the determinations of their vapor pres- 
sures, 


—— PUMP 
—— MANOMETER 


B Cc 


Experimental Part. 


The method of Rodebush and Dixon has been described 
in detail elsewhere.'!° ‘The apparatus in which the sub- 
stances were vaporized was made of pure nickel with welded 
joints by the American Nickel Corporation. ‘The modified [J 
form of the apparatus is shown in Fig.1. The reservoir A Fig. 1.—Appa- 
is 4.5 cm. in internal diameter and 2.5 cm. deep. The ‘tus central 
tubes B and C are 0.9 cm. in inside diameter. Instead of Sees 
constrictions in the tubes, loose plugs were introduced. ‘These plugs have 
a central hole 3 mm. in diameter for the ascent of the vapor and a slot E 
for the return of the condensed liquid. This proved to be a great improve- 
ment as it prevents clogging of the apparatus with drops of liquid. 

The apparatus was heated in an electric furnace with an insulating cover 
and the regulation was accomplished by a hand rheostat. ‘The tempera- 
ture did not vary by more than 0.2-0.8°. ‘Temperatures were measured 

4 Latimer, TH1s JouRNAL, 45, 2803 (1923); 48, 1234 (1926). 

5 Kroner, Ann. Physik, 40, 438 (1913). 

6 Von Wartenberg and Albrecht, Z. Elektrochem., 27, 162 (1921). Von Warten- 
berg and Schulz, ibid., 27, 568 (1921). 

7 Ruff and Mugdan, Z. anorg. allgem. Chem., 117, 147 (1921). 

8 Maier, Bur. Mines Tech. Papers, No. 360. 

9 Jackson and Morgan, J. Ind. Eng. Chem., 13, 110 (1921). 

10 Rodebush and Dixon, Phys. Rev., 26, 851 (1925). 


2524 E. F. FIOCK AND W. H. RODEBUSH Vol. 48 


by a platinum—platinum-rhodium thermocouple which was repeatedly 
calibrated against the boiling point of sulfur and the melting points of 
potassium chloride and potassium sulfate.1! ‘The apparatus was sealed 
to the glass pump and manometer system with de Khotinsky cement. 
Argon was used to furnish an inert atmosphere. Sulfuric acid was used in 
the manometer. The levels in the manometer were read to 0.02 mm. 
with a cathetometer and the density of the sulfuric acid was determined 
aftereachrun. About 25 g. of material was introduced into the apparatus 
for a determination. 

The boiling point of potassium in the neighborhood of one atmosphere 
was determined by boiling the metal in a nickel tube and measuring the 
temperature of the vapor with a thermocouple protected by a nickel 
shield. ‘The lower end of the tube was heated in an electric furnace and 
a heating jacket was placed around the tube above the liquid line to pre- 
vent excessive cooling of the vapor. Argon was used as an inert at- 
mosphere and the total pressure was obtained as the sum of the readings 
of a mercury manometer and the barometer. A constant temperature 
could be obtained in the vapor even while the rate of heating of the liquid 
and vapor jackets was varied considerably. 


Purity of Materials 


Mallinckrodt’s metallic potassium was used. It was freed from oil and oxide and 
introduced into the apparatus in a clean state by repeated filtering and distillation in a 
vacuum. Spectroscopic tests indicated less than 0.1% of sodium in the initial material. 
The salts used were recrystallized from products purchased as being of high purity and 
with one or two exceptions probably contained negligible amounts of impurity. ‘The 
sample of potassium bromide was furnished by Professor Braley of this Department. It 
had been recrystallized twice but was not known to be free from chlorides. ‘The cesium 
chloride was purchased as pure from Eimer and Amend, but it was not recrystallized. 
Tests showed it to be free from any considerable amounts of sodium and potassium. 
Tests on the material after a run showed that no appreciable amounts of nickel were 
dissolved. 


Results 


The results are given in the tables. In addition, an empirical equation 
has been fitted to each set of data. This equation is linear in log p and 1/T 
in every case. ‘This is not surprising in the case of the salts since the meas- 
urements cover a comparatively short range of temperatures. In the case 
of potassium, however, where the range is considerable, the absence of 
curvature is surprising since the difference in the heat capacities of the 
liquid and vapor must be two or three calories. ‘The authors have noticed, 
in fitting equations to vapor-pressure data for other metals, that a linear 
equation often seems to fit as well as one that contains a term for AC, 
and they are at a loss to explain this. It may be due to deviations of the 
saturated vapor from the perfect gas law at higher pressures. 

4 Roberts, Phys. Rev., 23, 386 (1924). 


Oct., 1926 SOME PROPERTIES OF POTASSIUM 2525 


Previous tests of the experimental method on substances of known vapor 
pressure, such as mercury, have indicated that the accuracy of the method 
is limited only by the uncertainty of the temperature control. The plot 
shows the data to be highly consistent and the absolute error of the meas- 
urements is believed to be less than 1% on the average. 


TABLE I 


VAPOR PRESSURES OF METALLIC POTASSIUM 


Pressure, mm. of Hg Pressure, mm. of Hg 


Temp., °C. Obs. Calcd. Temp., °C. Obs. Calcd. 
406.2 4.60 4.57 509.5 32.80 3352 
427.9 eee €.26 528.5 44.83 45.9 
448.6 11.18 11.0 754.3 744.0 741.7 
469.1 16.23 aul 63 757 0 763.1 q6L-2 
489.4 20.00 23.5 759.8 783.3 782.0 

login p = —4433/T + 7.1830 

AH = 20,260 cal. per gram atom 

TABLE II 


VAPOR PRESSURES OF SODIUM CHLORIDE 


Pressure, mm. of Hg Pressure, mm. of Hg 


enpr. G: Obs. Caled. ‘Lemp: sc Obs. Caled. 
976.5 6.12 6.19 1079.5 Zora 23.23 
1002.5 8.71 8.83 TOS Ae alae 31625 
1028.4 12:/37 12.38 113012 41.27 41.46 
1054.1 LieG7 17.09 1155.4 54.16 54.45 
logio p = —9419/T + 8.3297 
AH = 48,050 cal. per gram molecule . 
TaBLe III 


VAPOR PRESSURES OF POTASSIUM CHLORIDE 


Pressure, mm. of Hg 


Pressure, mm. of Hg 
O 


bs. 
24.85 
34.00 
45.39 
54.54 


Calcd. 

24.81 
33.69 
45.27 
54.75 


Pressure, mm. of Hg 


Obs. 
20.82 
25.69 
31.69 
38.68 
46.74 


Temp., °C. Obs. Calcd. Wemp., °C. 
906.0 4.30 4.19 1036.9 
932.0 6.26 6.15 1062.6 
959.0 9.03 9.01 1088.0 
985.2 12.84 12.85 1105.0 

iid es 18.09 17.98 

logio Pp = —9115/T + 8.3526 © 
AH = 41,660 cal. per gram molecule 
TABLE IV 
VAPOR PRESSURES OF POTASSIUM BROMIDE 
Pressure, mm. of Hg 

Temp: °C: Obs. Caled. ‘Temp:, -CG: 
906.0 6.32 6.32 993.9 
923.8 8.15 8.16 1011.1 
941.4 10.44 10.42 1028.4 
959.0 13.23 13.21 1045.6 
976.5 16.60 16.62 1062.6 


logio p = —8780/T + 8.2470 


AH = 40,130 cal. per gram molecule 


Caled. 
20.76 
25.70 
31.70 
38.81 
46.10 


E. F. FIOCK AND W. H. RODEBUSH Vol. 48 


TABLE V 


VAPOR PRESSURES OF POTASSIUM IODIDE 


Pressure, mm. of Hg Pressure, mm. of Hg 


Temp., °C. Obs. Caled. Temp., °C. Obs. Caled. 
842.9 5.29 5.27 950.2 23.31 23.38 
852.0 6.07 6.05 976.5 32.25 32.39 
879.0 9.00 8.98 1002.5 43.88 43.13 
897.0 11.538 11.56 1028.4 60.08 59.30 
923.8 16.62 16.62 

logio P = —8229/T + 8.0957 

AH = 37,610 cal. per gram molecule 

TABLE VI 


VAPOR PRESSURES OF CESIUM CHLORIDE 


Pressure, mm. of Hg Pressure, mm. of Hg 


aL ein Dec Obs. Calcd. Temp oc: Obs. Calcd. 
824.7 4.15- 4.30 959.0 28.60 28.54 
852.0 6.49 6.55 985.2 39.42 39.39 
879.0 9.77 9.74 1011.1 53.13 53.46 
906.0 14.18 14.238 1019.9 58.48 59.14 
932.0 20.32 20.18 
logio p = —8282/T + 8.1772 
AH = 37,854 cal. per gram molecule 
TaBLE VII 
DEVIATIONS OF CALCULATED VALUES FROM OBSERVED VALUES 
Total Max. dev., Av. dev., Max. dev., Av. dev., 
Substance observations mm. mm. 0 % 
K 17 met 0.71 3.3 ri 
NaCl 23 0.70 re | 137 0.5 
KCl 24 .40 .10 4.6 8 
KBr 10 64 .09 1.4 3 
KI 22 .78 .14 1.5 4 
CsCl 23 .34 .09 3.5 Tt 


Thermal Properties 


In Table VIII are shown the principal thermal data for the substances 
investigated. ‘The entropies of the solid at 298° K. have been calculated 
from specific-heat data except in the case of potassium iodide and cesium 
chloride, for which the values were estimated. ‘The heat of fusion of po- 


TABLE VIII 
HEATS OF SUBLIMATION 

K NaCl KCl KBr KI CsCl 
Ssolid 298°K- 16.5 17.6 19.8 22.6 »- S433 
Ty Sa6.6: > 10%o 1043 1001 950 918 
ASy <6 Gry 6.1 6.2 6:2 6.2 
: ASyaporization to 1 atm. at Ty 19 . 65 24 . 9 25 . 0 24 . 5) 23 . 8 24 . 2 
298 
frcdmT 0.24 “Str a3 6.1. 6.80 5.6 
Ty 
Svapor 298°K- 1 atm- 38.0 55.6 Dic 59.4 60.2 59.3 


Oct., 1926 SOME PROPERTIES OF POTASSIUM at 


tassium is given by Bernini,!* and for sodium and potassium chlorides 
by Plato.'* ‘The other heats of fusion have been estimated. ‘The heats 
of vaporization are calculated from our data by the Clapeyron relation. 
AC, is assumed to be —5 cal. 


TABLE IX 
HEATS OF SUBLIMATION 
Lattice Heat of soln. 
AH gublimation at 298°K- energy of gaseous ions 
NaCl 54 182 248 
KCl 52 163 235 
KBr 50 156 216 
KI 47 144 195 
CsCl 47 Berl 225 


culate its lattice energy. 


In Table IX are given the values in kilogram calories of the heats of 
sublimation of the crystals at 298°K. calculated by us, the values of the 
lattice energies as calculated by Born’ and the heats of solution of the 
gaseous ions calculated by Born’s’ formula 

AE = (Ne?/2r)[1 — (1/D)] (1) 
The figures for the lattice energies are the original calculations of Born 
and would be changed somewhat by new values of the constants involved. 
The calculation of the heat of solution involves the arbitrary choice of 
the “‘atomic radii.’” Born himself does not claim a high accuracy for his 
lattice energy value but apparently the only test of his theory that has been 
made with dependable data is the comparison made by Richards and 
Saerens!® of experimental compressibilities with those calculated by Born 
and here the agreement seems as good as one could expect. All other tests 
of his theory involve uncertain ionization potentials or ‘“‘electron affinities.” 
Latimer* has recently attempted to verify the formula for the heat of solu- 
tion of gaseous ions by the use of very uncertain data. It seems rather 
more plausible to assume that the formula is correct and draw what in- 
ferences we may. When we examine Table VIII we notice first a satis- 
factory parallelism between the values for the heat effects in the three 
columns. By far the most striking features, however, are the extremely 
small values of the heats of solution of the solid alkali halides in spite of 
the large values of the heats of vaporization and the lattice energies. ‘This 
means that the lattice energy is nearly equal to the heat of solution of the 

22 Bernini, Physik. Z., 7, 168 (1906). 

13 Plato, Z. phystk. Chem., 55, 737 (1906); 58, 369 (1907). 

14 Born, Verh. deut. physik. Ges., 21, 13 (1919). The lattice energy is the energy 
increase involved in the separation of the ions of the crystal to an infinite distance from 
one another. 

% Born, Z. Physik, 1, 45 (1920). 

16 Richards and Saerens, THIS JOURNAL, 46, 934 (1924). 


2528 E. F. FIOCK AND W. H. RODEBUSH Vol. 48 


gaseous ions or, in other words, that the electrical forces of the ion are neu- 
tralized to about the same extent in solution as in the crystal lattice. 
This would seem to justify Born’s tacit assumption that the energy relations 
of an ion depend only upon its charge and its ‘‘effective atomic radius.” 
If we assume the dielectric constant of water infinite in Equation 1 for the 
heat of solution of gaseous ions, the energy value given is not altered ap- 
preciably, but the expression becomes identical with that for the neutral- 
ization of the ions in question by the closest approach of an ion of opposite 
sign and equal radius. ‘This approximates the energy change in the con- 
densation of a gaseous ion in the lattice!’ and hence it appears plausible 
that the two heat effects should be so nearly equal. In the process of the 
hydration of an ion, water is not to be pictured as a homogeneous medium 
of high dielectric constant. Rather the process consists of the neutraliza- 
tion of the ionic charge by the more or less polar molecules. In Equation 
1 no correction is made for the radii of the water molecules nor for repul- 
sive forces, and hence it is not surprising that the values in Col. 3 are 
larger than in Col. 2. 

Latimer has pointed out the apparent lack of specific action between the 
ion and the solvent. It seems certain, however, that the tendency to co- 
ordinate the solvent molecules as auxiliary valence groups is a function of 
the charge and effective radius of the ion and hence a specific property. 
Likewise, the tendency of a solvent to become coérdinated is a function of 
the effective radius and the potential polarity of its molecules. 

A final point for comment on the data in Table VIII is the small value 
of the heat of sublimation of a salt compared to its lattice energy. ‘This 
must mean that when a molecule vaporizes from the lattice, the two ions 
approach more closely and the molecule becomes less polar. 

If we subtract the heat of sublimation from the lattice energy, we have 
the heat of ionization of the salt vapors. ‘The values in Col. 3 must cer- 
tainly represent a maximum value for the lattice energy, while we suspect 
the figures in Col. 2 to be near the right value. This would indicate that 
the heat of dissociation of sodium chloride vapor into sodium and chloride 
ions is in the neighborhood of 128 calories. 

Summary 

The vapor pressures of potassium and five alkali halides have been meas- 
ured. 

The thermal data have been calculated for these substances. 

Some inferences favorable to Born’s theory of lattice energy have been 
drawn. 

URBANA, ILLINOIS 
17 Neglecting repulsive forces the potential energy of the lattice is (0.145 e?)/ 


(r+ + r-) (where r+ and r- are the respective radii) per bond, per ion, and each ion has 
six bonds. ) 


VE TEL UN 


The writer was born at Olney, [linois, October 
17, 1902. At the age of four his primary in- 
struction was begun in Phoenix, Arizona. In 
1910 he entered the fourth grade of the public 
schools at Olney, [linois, and remained in those 
schools until his graduation from high school in 
1919. In the fall of that year he entered the 
University of Illinois, and received from that in- 
stitution the degrees of Bachelor of Science in 
Chemical Engineering in 1923 and Master of Sci- 
ence in Chemistry in 1924. For the term 1924- 
1925 he held a graduate scholarship in Chemistry, 
for 1925-1926 a quarter time assistantship in the 
department of Physical Chemistry, and for 1925- 
1926 a University fellowship in Chemistry. 


The summer months of 1924 and 1925 were 
spent at the United States Bureau of Standards. 


PUBLICATION 


W. H. RopesusH AND EH. F. Fiock. The Measurement of the Absolute 
Charge on the Earth’s Surface. Proc. Natl. Acad. Sci. II, No. 7, 402 (1925). 


112 07 


= 
30 2887372 Z 


